1. Field of the Invention
The present invention relates to high aspect ratio metal microstructures and methods for preparing such high aspect ratio metal microstructures.
2. Discussion of the Background
There are a variety of important military and industrial applications for high resolution metal microstructures with good adhesion on a variety of technologically relevant surfaces. These include interconnects and vias in silicon based microcircuits as well as high resolution conductive paths on printed circuit boards, packaging, microwave transmitters and receivers.
These are also a variety of application for high resolution metal microstructures with high aspect ratios. These include electron beam sources which make possible the fabrication of high power microwave devices, free electron lasers, projection electron beam lithographic sources and flat panel displays. Other applications for high resolution high aspect ratio structures include x-ray masks, self shielding interconnects, controlled release microvials, microelectrodes and scanning tunneling electron tips.
Recently there has been a large amount of interest in vacuum field emission from arrays of sharp tips (T. Utsumi, IEEE Trans. Elctron Devices, vol. 38, p. 2276 (1991)). This is because of their potential use in vacuum microelectronics (C.A. Spindt, etal, J. Appl. Phys., vol. 47, p. 5248 (1976)), flat panel displays (Fortune, Dec. 2, 1991, p. 132) high power switches (C.W. Roberson, Proc. Soc. Photo-Opt. Int. Eng., vol. 453, p. 320 (1983)), etc. Another geometry has recently been reported that makes use of metal-coated biologically derived cylinders for free electron laser applications (Kirkpatrick etal, Applied Phys. Lett., vol. 60, p. 1556 (1992); and Kirkpatrick et al, Nuclear Instruments and Methods in Phys. Res., Elsevier, NY, p.1, (1991)). Numerical modeling has shown that electron beam brightness for a hollow cylinder geometry should be superior to that of a sharp tip. However, problems with the biologically derived cylinders fabrication process severely limits their performance.
The Fowler-Nordheim, (Fowler et al, Proc. R. Soc. London A, vol. 119, p-173 (1928)) field emission current density, J.sub.FN, describes the process of quantum filed emission from a onedimensional cold-cathode system, ##EQU1## where A=1.54 X10 .sup.-6, B=6.87.times.10.sup.7, y=3.79.times.10.sup.-4 (.beta..EPSILON.).sup.1/2 /.phi.), t.sup.2 (y)=1.1, v(y)=0.95-y.sup.2, .beta. is the field enhancement factor due to local geometry, .EPSILON. is the applied electric field in V/cm, and .phi. is the work function in .theta.V of the surface emission material. Precise values for t.sup.2 (y) and v (y) are reported in the literature (Miller, J. Franklin Inst., vol. 282, p. 382 (1966) , and Miller, J. Franklin Inst., vol. 287, p. 347 (1969)).
Hollow cylinders can be used to produce a local enhance ment of the applied electric field whose magnitude is dependent upon cylinder height, the average spacing between nearest neighbors, the radius of curvature of the metal wall at the edge of the exposed hollow cylinder, and the nature of the surface near the exposed edge. Detailed numerical simulations (Kirkpatrick et al, Nuclear Instruments and Methods in Phys. Res., Elsevier, NY, p.1, (1991)) of the electrostatic field in the vicinity of a hollow cylindrical structure have shown that field enhancement factors in the range .beta.=150-250 may be readily achieved with a cylinder of diameter 0.5 .mu.m and a height h=10-15 .mu.m. The enhancement factor may be increased an additional 2- to 4-fold by the inherent surface roughness of the elctrolessly deposited metal film that makes up the outer cylinder surface, yielding an enhancement factor in the range .beta.=300-1000.
The hollow nature of the protruding tubule microstructures also provides an electrostatic lensing effect for the emitted electrons: the thinner the tubule wall, the greater the self-focusing effect of the structure, and the more collimated the emitted bean. For suitably fabricated structures, with thin wall thicknesses near the emission tip, normalized electron beam brightnesses well in excess of 10.sup.6 A/cm.sup.2 -rad.sup.2 can be achieved (Miller, J. Franklin Inst., vol 287, p.347 (1969)).
However, to date no approaches have proven suitable for producing field emitter arrays (FEAs) with the above structural and functional characteristics- In addition, no technique for the production of high aspect ratio metal microstructures has been demonstrated that has the ability to precisely control the height, diameter, center to center spacing, alignment, metal type, and metal thickness required for these devices.
In addition, it is desirable to -use x-rays as the source of actinic radiation in photolithographic techniques, because the short wavelengths of such radiation provides increased resolution. To fully realize the increased resolution potentially afforded by x-ray photolithography, masks which contain structural details on the order of about 10 to 0.01 .mu.m in width are desired. Further to permit the use of such penetrating irradiation, masks with a stopping power equivalent to about 1-3 .mu.m of a metal such as gold or nickel are required. However, to date masks which combine the desired fineness of structural detail and the required stopping power have not been available. High aspect ratio metal microstructures having heights of about 1 to 3 .mu.m and structural details having widths of 1 to 0.01 .mu.m, if available, would thus be useful as masks for x-ray photolithography.
Recently, the use of microtubules as carriers for the controlled release of active compounds, such as antifouling agents, pesticides, antibiotics, etc., has been reported. Thus, high aspect ratio metal microstructures which are in the form of tubules having one or two open ends would be useful as carriers for the controlled release of active agents. However, the production of such microtubules remains problematic, especially for those with only one open end.
In addition, the ability to prepare anisotropic high dielectric interconnects between one or more microcircuits on separate layers of a semiconductor or similar device with the desired packing of today's dense microcirucuits remains an elusive goal. If it were possible to precisely, place metal structures with heights of 0.5 to 5 .mu.m and widths of 0.5 to .about.4 .mu.m on the surface of a device, such metal structures could serve as highly anisotropic interconnects between a circuit on the layer on which the structures are placed and a circuit on a layer subsequently added with an increase in shielding of the electron field between two different circuits in close proximity. For a general discussion of metal features and pillars in multilevel interconnect metallization, see Vivek D. Kulkarni et al, J. Electrochem. Soc. Solide State Sci. and Tech., vol. 135, no. 12, pp. 3094-98, (1988).
It is also desirable to provide ultramicroelectrode arrays (UMAs) for use as sensors, e.g., in solid state electrochemistry. The benefits attending such electrodes have been discussed by Fleischmann et al, J. Phys. Chem., vol. 89, pp. 5537-5541 (1985). In particular, it is desired to prepare addressable ring microelectrodes with metal line width dimensions on the order of 500 to 1000 .ANG.. However, the production of such devices is currently unachievable. Penner et al, Anal. Chem., vol. 59, pp. 2625-2630 (1987) report the production of ensembles of ultramicroelectrodes. However, the electrodes are neither regularly spaced nor arranged substantially parallel to one another. Thus, if it were possible to prepare regularly spaced addressable metal microstructures with such dimensions, such metal microstructures could serve as UMAs.
Thus, there remains a need for high aspect ratio metal microstructures which would be useful as anisotropic interconnects, electron emitters, x-ray photolithography masks, ultramicroelectrode arrays, and carriers for the controlled release of active agents. There also remains a need for a method of producing such high aspect ratio metal microstructures.